Test the effectiveness in reducing new caries increment over a three year period of: a 0.5% sodium fluoride mouthrinse (0.0225% F) used daily in school; a 0.2% sodium fluoride (0.09% F) mouthrinse used daily in school; a 0.5% sodium fluoride mouthrinse used weekly in school; and a 0.2% sodium fluoride mouthrinse used weekly in school. To compare the effectiveness of the different concentrations and frequencies.